The present invention relates to dampers for bearings to apply braking forces to the shaft system of a gas turbine engine. More specifically, the present invention relates to damping using electrical machine concepts.
Dampers have been commonly used to apply braking forces to the orbiting shaft system of a gas turbine. The gas turbine may, for example, be an aircraft engine. Often the bearing dampers dump damping energy into a damping medium such as a gas or liquid. The gas or liquid which serves as the damping medium must be kept sufficiently cool to avoid interfering with the turbine operation. In the case of a gas damper, the required cooling may simply occur by the thru-flow of the gas. A hydraulic damper using liquid may require additional arrangements to provide sufficient cooling of the hydraulic fluid for proper operation.
Various electromagnetic active bearing dampers have been developed to compete with gas or hydraulic bearing dampers. These electromagnetic active bearing dampers have been magnetic attraction force dampers and eddy current dampers.
The force densities developed in magnetic attraction dampers are in the order of 200 to 300 lbs. per square inch (PSI), whereas the eddy current dampers have had force densities on the order of 1 to 4 PSI. Response time for the magnetic attraction dampers is relatively low because of the large inductance to resistance ratio for the excitation coil. A full force buildup may take from 20 to 200 milliseconds. The eddy current dampers can have full force build up in the range of 10 to 100 microseconds. Magnetic attraction dampers have practically no heat input into the moving part, whereas eddy current dampers can have up to 50 watts per square inch heat input. In other words, the low heat input to the moving part in a magnetic attraction damper has to be weighed against the slow response time, whereas the fast response time in the eddy current arrangement has to be weighed against the disadvantage of high heat input into the moving part.
A disadvantage common to numerous active bearing designs that the energy removed from the braking of the engine will then be wasted. In other words, the braking of the rotating shaft system produces only waste energy.